Inmarsat is an international organization providing satellite communications services to ships at sea, land-mobile operations, and aircraft. SATCOM communication links generally provide for telephone services (e.g., spoken voice, PC data, and FAX communications) and packet data communications (e.g., computer-to-computer communications via X.25 protocol). The SATCOM concept for Inmarsat is to bring airborne intelligent communications services to aircraft.
Referring to FIG. 1, Inmarsat SATCOM communications network 10 includes an Aircraft Earth Station (AES) 12 and a Ground Earth Station (GES) 14 in communication with a satellite 16 via radio links 18 and 20, respectively. GES 14 communicates with a plurality of user devices 22 via a Packet Switched Data Network (PSDN) 24 and a Packet Switched Telephone Network (PSTN) 26. User devices 22 include, for example, a telephone 28, a facsimile (FAX) machine 30, and a modem-equipped personal computer (PC). Telephone 28 can be any type of voice communication system including cellular phones, PHY phones, digital phones, analog phones, as well as other voice communication devices. PSDN 24 and PSTN 26 are telecommunications networks which provide international ground communications services and can include any type of ground communication services.
AES 12 includes a SATCOM communications terminal mounted within an aircraft. The term "Earth Station" in AES 12 is indicative of the aircraft being an earth station with respect to satellite 16 since the aircraft is not high above the earth compared to the altitude of satellite 16 above the earth. AES 12 provides telephone, FAX and PC data communications services for both aircraft crew and passengers. AES 12 also provides in-flight entertainment and special services such as broadcast news briefs, reservations and catalog sales.
For simplicity, FIG. 1 shows only one AES 12, one GES 14 and one satellite 16. Network 10, however, may include more than one AES 12, GES 14 and satellite 16. For example, multiple aircraft may communicate with each satellite 16, multiple satellites 16 provide coverage for multiple regions around the earth, and multiple GES 14's may operate in the region covered by each satellite 16. Similarly, user devices 22 may include more than one telephone 28, FAX machine 30 and/or PC 32.
Referring to FIG. 2, link 18 between AES 12 and satellite 16 may include six radio channels with three data channels 34-38 and three circuit channels 40-44. All six channels communicate digital data. Data channel 34 radiates or communicates from satellite 16 to AES 12 and is referred to as the P Channel, and data channels 36 and 38 radiate from AES 12 to satellite 16 and are referred to as the R and T Channels, respectively. Channels 34-38 communicate packet data traffic between computers. Circuit channels 40-44 radiate in both directions between satellite 16 and AES 12, and are referred to as C Channels. Circuit channels 40-44 communicate regular telephone calls (e.g., generically referred to as "voice channels" herein, even though FAX or PC modem data may also be communicated).
P Channel 34 is radiated continuously, and is used to communicate data needed for call setup. P Channel 34 is known as the "forward link" since it links up to the aircraft. R and T Channels 36 and 38 are the return links for communicating data (e.g., aircraft positioning data, passenger fax data, etc.) back to satellite 16. Each AES 12 which communicates via the same satellite 16 uses the same frequencies for P, R and T Channels 34-38.
In contrast, C Channel frequencies are unique for each particular telephone call. A particular C Channel frequency pair is assigned to each telephone call. When the call terminates, the frequency is returned to a pool so it can later be re-assigned to another telephone call. "Circuit channel" originally referred to the copper link between telephones in old telephone networks, but is used herein to refer to a telephone circuit channel or connection generally.
FIG. 2 shows a four-channel AES 12. The signal in each channel type is processed by a channel unit. Thus for a four-channel AES, there is one channel unit for packet data communications (i.e., P, R/T) and three for circuit mode communications. Each channel unit consists of both transmitter and receiver circuitry. The data and C Channel units are identical in circuitry but differ in software programming. Configuration of the channel units may depend on the aircraft application. If an application does not require the communication of data packets (i.e., "circuit-mode services only"), the P, R and T Channel Unit can be configured as a circuit channel instead. For example, AES 12 would be configured to provide four C Channels for telephone communications if the P, R and T Channels were not needed for communicating data traffic. However, as explained below, conventional AES terminals must continue to reserve a data channel unit 34 as a data channel.
During operation, AES 12 and satellite 16 move with respect to ground (i.e., GES 14) and with respect to each other. Commercial passenger and cargo aircraft typically cruise at ground speeds exceeding several hundred miles per hour. Satellite 16 also moves with respect to ground. The relative movement between AES 12 and satellite 16, and between satellite 16 and GES 14, induces a Doppler shift in the frequency of radio signals being transmitted between these components.
Compensation for Doppler frequency shift is applied to correct the signals to maintain the signals within the system frequency error budget specified by Inmarsat. Exceeding this error budget may cause adjacent signals to interfere with each other. It is relatively easy for AES 12 to compensate for Doppler shift on received signals by listening to incoming signals and adjusting the frequency of the receiver appropriately. However, it is difficult for GES 14 to compensate for Doppler shift in received signals since GES 14 may need to communicate with many aircraft in its region, so that GES 14 would be required to track Doppler shift separately for each aircraft.
To solve this problem, Inmarsat requires that each AES 12 transmit all of its signals to satellite 16 within a defined frequency tolerance (e.g., 100 or 185 Hz). To meet the requirement, AES 12 listens to signals received on P Channel 34, which are continuously available, and measures the frequency error between received P Channel signals and the known P Channel transmission frequency. It is assumed that this error is caused by Doppler shift. AES 12 uses the measured error to determine an expected Doppler shift for each transmission based upon the ratio of the P Channel receive frequency to the R, T and C Channel transmit frequencies, and applies the expected Doppler shift to correct the transmissions on the R, T and C Channels to compensate for the Doppler shifts. Thus, the frequency error budgets will not be exceeded.
No resources are wasted if the aircraft application has a data communications requirement since the P Channel used for monitoring Doppler shift is needed for other purposes. If, however, an application does not require data communications (i.e., circuit-mode services only), AES 12 does not need a channel allocated for receiving P Channel signals for use in communicating data packets. However, in this situation, conventional AES terminals continue to reserve one channel for receiving P Channel signals for the sole purpose of monitoring Doppler shift. P Channel signals are conveniently used for this purpose since these signals are continuously available.
Thus, in a circuit-mode services only application, one channel of the four-channel AES terminal shown in FIG. 2 would continue to be configured as a data channel for the sole purpose of measuring Doppler shift based on the frequency of the P Channel inputs. Only the other three channels could be used as circuit channels. A customer purchasing a SATCOM radio terminal capable of supporting five voice telephone calls must therefore purchase a six-channel terminal since one channel must be reserved for monitoring P Channel traffic. In general, customers who need only circuit-mode services are forced to order an AES terminal equipped with one more channel than the number of telephone calls the system must support. The additional channel needed is a costly, wasted resource.
One method that has been used in an attempt to address this problem is to pre-compensate the transmit frequencies based on a calculated Doppler shift rather than a measured Doppler shift. The method calculates the Doppler shift using the satellite location, the aircraft location, and data which defines flight characteristics (e.g., velocity, ground speed, heading pitch, roll, etc.). Much of the data needed to calculate the Doppler shift comes from the aircraft's Inertial Reference System (IRS). This method has been difficult to implement due to the many different types of IRS systems which are made by many manufacturers and provide inconsistent flight data. Thus, it has been difficult to produce an AES terminal which calculates Doppler shift and is compatible with different aircraft.
Accordingly, it would be advantageous to provide an improved AES terminal or communication system which can be used in circuit-mode services only applications and is not required to reserve a channel unit solely for monitoring P Channel input signals. Customers of this AES terminal would not need to buy a radio terminal equipped with one more channel unit than the number of circuit channels actually needed. It would also be advantageous to provide a multichannel terminal for satellite communications wherein transmissions remain within a specified frequency error budget even with each channel configured as a circuit channel. It would also be advantageous to provide a method for measuring Doppler shift in an AES terminal or communication system independent of P Channel inputs.